Hereinafter, a conventional electronic part will be described.
In the prior art, a surface acoustic wave device (hereinafter, called as “SAW device”) is described as an example of an electronic part.
In recent years, a compact and lightweight SAW device is prevalently used for electronic equipment such as various types of mobile communication terminal equipment. In particular, in a radio circuit section of a mobile phone system within a band of 800 MHz to 2 GHz, an SAW filter has been widely used. Such an SAW filter is formed by a lithium tantalate (hereinafter, called as “LT”) substrate which is cut of a Y-axis at a cut angle of 36° as its rotational angle around the X-axis in the Z-axis direction, or a so-called 36° Y-cut X-propagation LT (hereinafter, called as “36° YLT”) substrate. However, depending on a place at which a filter is used in a mobile phone system or its radio circuit section, filter characteristics are required, such as a small insertion loss within a further passing band and a steep skirt property of the filter, as well as a high suppression level within a block band. In order to meet such demands, there is a method in which an LT substrate is used that is cut of a Y-axis at a cut angle of 42° as its rotational angle around the X-axis in the Z-axis direction, or a so-called 42° Y-cut X-propagation LT (hereinafter, called as “42° YLT”) substrate is used. According to this method, an SAW filter can be realized which has a smaller loss and a steeper filter skirt property than those of the conventional 36° YLT substrate. Such a method is described in Japanese Unexamined Patent Publication No. 9-167936.
However, similarly to the conventional 36° YLT substrate, such a 42° YLT substrate has a great thermal expansion coefficient in the direction where a surface acoustic wave is propagated. Besides, the elastic constant itself varies according to the temperature. Thereby, the frequency characteristics of a filter may also be largely shifted by approximately −36 ppm/° K. according to a change in the temperature. This is disadvantageous in temperature coefficient. For example, taking a PCS (Personal Communications Service) transmission filter from the United States into account, its center frequency of 1.88 GHz at the room temperature changes by about ±3.3 MHz or some 6.6 MHz within a range of ±50° C. at the room temperature. In the case of the PCS, there is an interval of only 20 MHz between its transmission band and reception band. Hence, also considering the dispersion of frequencies in its production, in practice, the transmission and reception interval is only substantially 10 MHz for the filter. Thus, for example, if the transmission band is desired to be secured over the entire temperature (i.e., within the whole range of ±50° C. at the room temperature), an adequate attenuation value cannot be obtained on the reception side.
In addition to the above, an improvement on the temperature coefficient is provided by forming an insulating film on a comb-shaped electrode. Normally, the shape of the comb-shaped electrode appears as the shape of the insulating film. Accordingly, unwanted reflection occurs at edges of concave portions of the insulating film, which may degrade electric characteristics of the filter.
Patent documents Japanese Unexamined Patent Publication No. 9-167936 and International Patent Publication Pamphlet No. 96/04713 are known, for instance, as prior art document information relating to the invention of this application.